Proper developmental processes require a tight control of spatial and temporal gene regulation, since specific gene and protein expression is a prerequisite of cell differentiation. Transcription factors as well as microRNAs are major components for transcriptional and translational control of gene expression. In Arabidopsis thaliana, one of the plant specific transcription factor families is the SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) gene family, which comprises 17 members, that have been shown to play important roles in several developmental processes. A decisive step in plant development is the transition from vegetative to reproductive growth, as it has to happen during favorable conditions to ensure successful reproduction and is a "one-time decision", as this phase change is not reversible in the annual plant Arabidopsis. The MIR156/157-controlled SBP-box transcription factor SPL3 has been shown to play a role during flowering in Arabidopsis, since its constitutive overexpression in a microRNA insensitive form results in early flowering plants, that nevertheless remain photoperiodically sensitive. Moreover, it has been shown that SPL3 binds in vitro to the sequence core motif CGTAC. During this thesis the role of SPL3 during the development to reproductive growth should be elucidated through identification of target genes. The results of this work suggest the floral meristem identity gene FRUITFULL (FUL; AGL8) to be a direct target of SPL3. Expression studies of two transgenes carrying the reportergene GUS in combination with genomic FUL or the FUL promoter region revealed a precocious activation of FUL in cotyledons and leaves in an SPL3OX background. Moreover, these data indicate that both, the binding motifs in the promoter as well as in the FUL first intron, are required for proper activation of FUL. A global expression analysis revealed that sugar metabolism, red light signaling and the circadian clock are affected by overexpression of SPL3. Subsequent analysis of diurnal expression of clock genes as well as of leaf movement in SPL3 overexpressing plants revealed a shortened period of the circadian clock and a precocious activation of so called "evening genes"
